shooting bags

What is the best type of bag to use for the rear of the gun. I'm talking about the small one you squeeze to raise your aim or relax to lower your aim. What are they called and which is the best one? Where can I order one of these?

You know what my favorite bags are? Homemade ones out of old bluejean pants legs and filled with sand. Sew up one end really tight. Fill about 3/4 full of sand, then sew up the other end. They work just fine. 2 in the front, 1 in the back. That's how I shoot all my groups, unless I decide to take the Caldwell tackdriver bag for the front, and use a sandbag for the rear.